Dielectric Function of AlN Grown on Si (111) by MBE

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Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society

AIN FILM PROPERTIES BY X-RAY DIFFRACTION, RUTHERFORD BACKSCATTERING, AND ATOMIC FORCE MICROSCOPY Since the AIN films are grown on Si (111), the crystal structure template defined by the substrate encourages growth of wurtzite AIN with the hexagonal axis along the growth direction. This is confirmed by a 9-29 x-ray diffraction scan using a D-MAXB single-axis goniometer with a Rigaku RU-200BH rotating-anode x-ray generator, see Fig. 2 (a). Because of the substrate miscut, which cannot be compensated by our experimental arrangement, the Si (111) peak is broad and weak. The AIN (0002) peak for film #1 is found at 29=36.05' corresponding to a lattice constant c=4.98 A, in agreement with the literature for bulk AIN [1]. Therefore, the strain in the film due to the lattice mismatch between AIN and Si is completely relaxed by misfit dislocations. The width of the 20 peak is 0.3-0.4', slightly larger than the instrumental resolution of 0.2'. A rocking curve with the detector fixed at 2&=36.05' shows a peak at 16.2', indicating a tilt

between the c-axis of the film and the surface normal because of the substrate miscut. The magnitude of the tilt cannot easily be determined with our goniometer. The width of the AIN (0002) rocking curve peak, see Fig. 2 (b), is slightly less than 10, giving an estimate of the mosaic spread due to dislocations. The width of the AIN (0004) rocking curve peak is 0.86' (not shown). Film #2 shows similar x-ray diffraction results. Atomic force microscopy (AFM) analysis of the AIN films were carried out on a Dimension 3000 instrument in tapping mode. The surface roughness of the two AIN films was substantially different. Whereas the film #2 with mixed 2-D and 3-D growth mode had a surface roughness of about 142 A, film #1 with 2-D growth mode was much smoother and had surface roughness of about 10 A. Shown in Fig. 1 is a perspective view AFM image of the AIN film #1 evidencing the high growth quality. The film exhibits a clear terrace and step structure that is typical of films with layer-by-layer 2-D growth on tilted substrates. The angle subtended by the terraces with the average film surface is about 2'. Assuming that the terraces are the AIN (0001) planes the miscut angle for AIN would be 20 which compares favorably with the intended miscut of the Si substrate of 3'. The average step height measured by AFM is greater than 30 A. Since the thickness of 1 ML of AIN is 2.49 A, the AFM results show that there is significant step bunching during AIN growth resulting in steps that are greater than 10 MLs high. Rutherford backscattering (RBS) finds that the density of the AIN film #1 is 3.0 g/cm 3 , about 10% lower than the published density of 3.26 g/cm 3 [1]. For film #2, the density is 2.9 g/cm 3 , i.e., somewhat lower. Most likely, the differences in density are due to hydrogen (possibly saturating dangling bonds at structural defects) or other impurities (such as oxygen) propagating along dislocations or other growth defects (p